The present invention relates to friction stir welding machines. More particularly, the invention relates to calibration techniques and devices for friction stir welding machines.
Friction stir welding is a process of welding two or more workpieces together and/or repairing cracks in a single workpiece using friction heat generated by a rotating pin tool. The pin tool is inserted into a joint between the workpieces (or sections of a single workpiece) and then rotated and moved along the joint to generate friction heat that forms a plasticized region along the joint.
A typical self-reacting friction stir welding pin tool includes an upper shoulder and a lower shoulder that together sandwich the workpiece or workpieces to be welded. The upper and lower shoulders can be moved toward or away from one another by actuators to accommodate workpieces of different thicknesses and to apply forging forces and pinch forces to the workpieces. The frictional heat generated by the pin tool is a function of the total forging forces of the upper and lower shoulders. Thus, it is important to accurately measure and control the applied forge forces to optimize various aspects of a welding operation.
Conventional friction stir welding machines typically measure forge forces indirectly with force transducers, pressure sensors, displacement sensors, or other devices that measure the amount of axial force applied by the actuators. Because of friction and other losses, the amount of force applied by the forge actuators doesn't always equal the actual force exerted on the workpieces. Moreover, such transducers, sensors, etc. often “drift” over time. It is therefore necessary to occasionally calibrate these transducers, sensors, etc.
Known calibration techniques use a sensor or sensors placed between a test workpiece and the upper and lower shoulders to measure the actual amount of force applied to the workpiece. These actual forces are then compared to the forces measured by the friction stir welding machine's internal transducers or sensors and any discrepancies are used for calibration. Unfortunately, these calibration techniques don't permit the upper and lower shoulders to be rotated during testing, resulting in force measurements that don't always equal the forces experienced during rotation of the shoulders due to friction in the head mechanism.
Accordingly, there is a need for improved friction stir welding calibration techniques that overcome the limitations of the prior art.